Iraqi government forces and allied tribal militias have started an all-out offensive to push al-Qaida-linked militants from parts of a city west of Baghdad, officials say.

Since late December, members of an al-Qaida offshoot - known as the Islamic State of Iraq and the Levant - have taken over parts of Ramadi, the provincial capital of the largely Sunni western province of Anbar.

They also control the centre of the nearby city of Fallujah, along with other non-al-Qaida groups that also oppose the Shia-led government.

The military officer said fierce clashes are taking place, but gave no details.